Int Journ Earth Sciences (1999) 88:253–275 Q Springer-Verlag 1999 ORIGINAL PAPER O. Kempf 7 A. Matter 7 D. W. Burbank 7 M. Mange Depositional and structural evolution of a foreland basin margin in a magnetostratigraphic framework: the eastern Swiss Molasse Basin Received: 11 March 1998 / Accepted: 12 March 1999 Abstract This integrated study of the sedimentology, Key words Molasse Basin 7 Alpine tectonics 7 magnetostratigraphic chronology and petrography of Alluvial sedimentation 7 Magnetostratigraphy 7 the mostly continental clastics of the Oligocene to Foreland basin evolution Miocene Swiss Molasse Basin underpins a reconstruc- tion of facies architecture and delineates relationships between the depositional evolution of a foreland-basin Introduction margin and exhumation phases and orogenic events in the adjacent orogen. A biostratigraphically based high- The Molasse Basin is a classical foredeep at the resolution magnetostratigraphy provides a detailed northern side of the Alps. Displaying a highly asymme- temporal framework and covers nearly the whole strati- trical cross section with a thick clastic succession, it graphic record of the Molasse Basin (31.5–13 Ma). exceeds 4 km in the south and feathers out towards the Three transverse alluvial fan systems evolved at the north (Fig. 1). It is widely accepted that the structural southern basin margin. They are characterized by development of the orogenic wedge controls the geom- distinct petrographic compositions and document the etry and shape of the foreland basin and therefore has exhumation and denudation history of the growing strong influence on the facies distribution within the eastern Swiss Alps. Enhanced northward propagation basin (Beaumont 1981; Jordan 1981); hence, the strati- of the orogenic wedge is interpreted to have occurred graphy of a foreland basin reflects the tectonic evolu- between 31.5 and 26 Ma. During the period 24–19 Ma, tion of the adjacent orogen. intense in-sequence and out-of-sequence thrusting took The stratigraphy and the facies relationships of the place as Molasse strata were accreted to the orogenic Molasse have been studied intensely over the past wedge. A third active tectonic phase, possibly caused decades (e.g., Homewood and Allen 1981; Allen et al. by backthrusting of the Plateau Molasse, probably 1985; Berger 1985; Diem 1986; Keller 1989; Schoepfer occurred between ca. 15 and 13 Ma. Fan head migra- 1989; Schlunegger et al. 1993), but the causal linkages tion between 31.5 and 13 Ma is probably controlled by between the tectonic evolution of the Alps and the stra- the structural evolution of the thrust front due to tigraphic record in the basin (Homewood et al. 1986; Molasse accretion and backthrusting. Pfiffner 1986; Sinclair et al. 1991; Sinclair and Allen 1992) have remained obscure due to uncertainties in the chronological record. Now a newly established high-resolution magnetostratigraphy in the Swiss Molasse Basin provides a very detailed chronostrati- O. Kempf (Y) 7 A. Matter graphic framework (Burbank et al. 1992; Schlunegger Geologisches Institut, UniversitÄt Bern, Baltzerstrasse 1, et al. 1996; Kempf et al. 1997). Based on this, Schlun- CH-3012 Bern, Switzerland egger et al. (1997b) have established a close temporal e-mail: oliver.kempf6geo.unibe.ch, Tel.: c41-31-6318759, link of the relationships between the Alpine orogeny Fax: c41-31-6314843 and the Molasse Basin evolution on a transect across the central Swiss Molasse Basin. D. W. Burbank Department of Geosciences, Pennsylvania State University, The aim of this study is to reconstruct the deposi- University Park, PA 16802, USA tional and structural evolution of the Molasse of M. Mange eastern Switzerland. In particular, we use high-resolu- Department of Earth Sciences, University of Oxford, tion magnetostratigraphy to tie the sedimentary, struc- Parks Road, Oxford, OX1 3PR, England tural, and geometrical evolution of the proximal 254 Fig. 1 Geological bedrock Hegau map of the Swiss Molasse Ma NS Volcanics Basin and schematic strati- 14 Serra. OSM II graphy and age of the eastern Lg. Lake Constance Molasse and North Helvetic 18 I OMM Basel Tabular Jura Flysch (NHF) in a III Burdigal. Miocene Zürich north–south cross section. II Folded Jura USM I–III and OSM I–II 22 Aquitan. refer to informal sub-units USM (see text). The study area is 26 N a p p e s I outlined Chattian 30 Oligocene Bern Rupelian UMM 34 N NHF Eoc. Priabon. 50 km Siltstones Conglomerates Marls Sandstones Study Area Lake Geneva (Figs. 2, 3) A l p i n e Upper Freshwater Molasse (OSM) Folded Jura Crystalline Cores Upper Marine Molasse (OMM) Genève Helvetic Tertiary Lower Freshwater Molasse (USM) Lower Oligocene of Lower Marine Molasse (UMM) Molasse Switzerlandbasin the Rhine Graben and Flysch modified after KELLER et al. (1990) and SCHLUNEGGER et al. (1996) Molasse. Different dispersal systems and their catch- gression and consists of shallow-marine sand- and silt- ment areas were identified using heavy mineral suites stones of the OMM interfingering with large fan deltas of sandstones and the clast composition of conglomer- at the southern basin margin (Keller 1989; Schaad et al. ates. The geometry and the location of these dispersal 1992; Bolliger et al. 1995), followed by fluvial clastics of systems in the basin were mapped to reconstruct shifts the OSM (Burdigalian to Serravallian). These lithostra- of depositional systems. The combined information tigraphic units, however, are strongly heterochronous about the stratal architecture in the basin and the struc- both from west to east and from south to north (Diem tural evolution in the source terrain allows an improved 1986; Keller 1989; Schlunegger et al. 1996). understanding about the relationships between Following Schlanke (1974) and Kempf et al. (1997), thrusting and exhumation in the Alps and the sedimen- the Molasse units of eastern Switzerland are informally tary processes in the Molasse Basin. subdivided into subunits that are based on the heavy mineral content of the sandstones and, if obtainable, the clast composition of the conglomerates: USM I Geological setting (Rupelian-Chattian), USM II (Aquitanian), USM III (lower Burdigalian), OSM I (Burdigalian, continental Five lithostratigraphic units classically subdivide the equivalent to the marine OMM), and OSM II North Alpine Foreland Basin (Matter et al. 1980; (Langhian-Serravallian; Fig. 1). The subunit USM III is Keller 1989; Sinclair et al. 1991) for which we use the only of local significance (eastern Swiss Molasse; German abbreviations; these are the North Helvetic Kempf et al. 1997) and corresponds temporally to the Flysch (NHF), the Lower Marine Molasse (UMM), the lowermost OMM of central Switzerland (basal Luzern Lower Freshwater Molasse (USM), the Upper Marine Formation of Keller 1989). Molasse (OMM), and the Upper Freshwater Molasse The study area is located in eastern Switzerland (OSM; Fig. 1). The Molasse deposits form two shal- between Lake Zurich and the Rhine valley and covers lowing-, coarsening-, and thickening-upward megase- the proximal part of the Molasse Basin, including the quences. The first megasequence consists of the deep Subalpine Molasse as well as the southern part of the marine NHF (Priabonian), the marine UMM (Rupe- Plateau Molasse (Figs. 1, 2). Here, the Molasse deposits lian) and the USM (Rupelian to lower Burdigalian), a are well exposed and provide excellent insight into the thick succession of fluvial clastics (Fig. 1). The Burdi- tectonic structure of the southern basin margin (Fig. 3). galian age of the youngest USM deposits in the study Two major thrust sheets make up the northern and area (Kempf et al. 1997) contrasts with the western part eastern part of the Subalpine Molasse (Kronberg and of the Swiss Molasse Basin, where the youngest GÄbris thrust sheets, Fig. 2C). The southern area is deposits of the USM comprise Aquitanian strata more disrupted and consists of two larger thrust sheets (20 Ma; Berger 1985; Schlunegger et al. 1996). The (Schorhüttenberg and Speer thrust sheets; Fig. 2C) and second megasequence starts with the Burdigalian trans- three smaller, internally disturbed thrust sheets 255 (Habicht 1945a). The northward dip of the upwarped derived directly from the thrust front (Habicht 1945a). strata of the Plateau Molasse is interpreted to be We add a marine depositional system that comprises caused by a south-vergent backthrust as a consequence all marine deposits of UMM and OMM (Fig. 2B). of imbrication of the southward-dipping thrust sheets These deposits include offshore marls and coastal sand- of the Subalpine Molasse (triangle zone; StÄuble and stones of the UMM (Frei 1979; Diem 1986) as well as Pfiffner 1991). shallow-marine silt- and sandstones of the OMM (Keller 1989). Methods Sedimentary petrography Depositional systems For the identification of different dispersal systems, Based on detailed sedimentological logging in the sandstones were sampled along the magnetostrati- Subalpine Molasse (USM; Kempf 1998) and the OSM graphic sections and analyzed for the heavy mineral lithofacies classification of Bürgisser (1981), five depo- composition according to Füchtbauer (1958, 1964) and sitional systems define the megascopic architecture of Matter (1964). Data published by Hofmann (1957) and the alluvial Molasse deposits (USM, OSM) according Füchtbauer (1964) were included to complete the to Schlunegger et al. (1997c; Fig. 2B). petrographic record. We quantitatively studied the clast Floodplain depositional systems commonly consist composition